1. Field of the Invention
The present invention relates to a valve driving system, and, more particularly to a valve driving system having an improved valve with stepwise driving force.
2. Related Art
It has been common to use a solenoid valve for opening and closing a passage. The solenoid valve, using an electromagnet for driving a valve body contained in a valve case, energizes and de-energizes current to the electromagnet, thereby moving the valve body between the valve-close and valve-open positions to open and close the passage. Solenoid valves are divided into two types: normally-open and normally-closed types. In the normally-open type, a return spring in the valve case maintains the valve body in the valve-open position when the valve is de-energized, whereas in the normally-closed type, the return spring maintains the valve body in the valve-close position when it is de-energized.
In the prior configuration explained above, when the valve body is driven from the valve-open position to the valve-close position to close the valve, the electromagnet is energized in the normally-open type (de-energized in the normally-closed type). A driving force as a resultant force of the electromagnetic force of the electromagnet and a spring force of the return spring (the spring force only in the normally-closed type) drives the valve body to the valve-close position. In this case, since the electromagnetic force of the electromagnet in the normally-open type (spring force of the return spring in the normally-closed type) is relatively large to ensure sealing (in other words, contact force between the valve body and the valve seat) of the closed valve, the driving force applied to the valve body is excessively large. As a result, a collision speed of the valve body to the valve seat becomes excessively high and an operational noise is caused by the collision between the valve body and the valve seat.